This invention relates to resid hydrotreating and, more particularly, to an improved hydrotreating process and novel ebullated bed reactor equipped with a vapor collector.
In the past, spiralling oil costs, extensive price fluctuations, and artificial output limitations by the Cartel of Oil Producing Countries (OPEC) have created instability and uncertainty for net-oil consuming countries such as the United States. It is desirable to attain adequate supplies of high-quality, low-sulfur, petroleum crude oil (sweet crude) from Saudi Arabia, Nigeria, Norway, and other countries at reasonable prices for conversion into gasoline, fuel oil, and petrochemical feedstocks. In an effort to stablize the supply and availability of crude oil at reasonable prices, Amoco Oil Company has developed, constructed, and commercialized extensive refinery projects to process poor quality, high-sulfur, petroleum crude oil and demetallate, desulfurize, and hydrocrack resid to produce high-value products, such as gasoline, distillates, catalytic cracker feed, metallurgical coke, and petrochemical feedstocks. Thus, it is Amoco Oil Company's goal to provide adequate supplies of gasoline and other petroleum products at reasonable prices.
During resid hydrotreating, residual oil (resid) is upgraded with hydrogen and a hydrotreating catalyst in a three-phase mixture of oil, catalyst, and vapor or gas bubbles to produce more valuable, lower-boiling liquid products. In order to increase the efficiency, effectiveness, and profitability of resid hydrotreating, it is desirable to maximize the conversion of resid to more valuable lower boiling liquid products. The extent of conversion of resid to more valuable lower-boiling liquid products depends in part on the residence time of the resid in the reactor, and the effective volume of the reactor itself.
In ebullated (expanded) bed reactors, the reactor oil and catalyst bed are fluidized, ebullated, and expanded. Since the reactor oil does not usually have enough velocity to expand the catalyst bed above its settled level, the reactor oil is recycled in the reactor, such as through a downcomer via an ebullating pump. During recycling, excessive gas (vapor) can be entrained in the recycled reactor oil. This can lead to high internal recirculation of gas which can cause high gas holdup and gas (vapor) bubbles in the reaction zone, impair operation of the recycle pump, and create other problems as explained below.
In the normal operation of ebullated bed reactors, there are substantial amounts of vapors or gas bubbles comprising excess hydrogen, light hydrocarbon gases, hydrogen sulfide, ammonia, steam, etc. emitted in the reaction zone. Excess vapors and gas bubbles can lower process efficiency, decrease product yield and cause various problems. First, they occupy a substantial portion of the overall volume of the reaction zone of the ebullated bed reactor that could be more effectively used to upgrade the oil feed. Second, a substantial portion of the liquid and vapors (gas bubbles) are recycled and recirculated through the downcomer and the ebullating pump, lowering pump efficiency and compounding the problem of occupying an even greater proportion of the overall volume of the reaction zone. The vapor and gas bubbles can also interrupt the smooth operation of the ebullating pump. The prior art reactors and processes do not solve this problem. Furthermore, vapors and gas bubbles in the oil feed materially decrease the capacity and effective volume of ebullated bed reactors, thus decreasing hydrotreating efficiency, prolonging the process in which it takes to produce upgraded oil, and adding to overall costs of producing oil.
Additional unsolved problems plaguing prior art reactors and processes are gas entrainment, gas holdup, and gas recycle. High gas entrainment causes the ebullating pump to be run at very high pump speeds, which constrains reactor operation. Recycle flow instability occurs at higher gas and liquid recycle rates where gas entrainment and internal gas recycle rates are high. High gas recycle may cause rapid changes in gas holdup in both the ebullated bed reactor and the downcomer. When this takes place, the pump inlet and outlet pressures vary widely and make it difficult to control flow rates and avoid catalyst slumping or carry over. Gas entrainment and the resulting internal gas recycle increase gas holdup and reduce the reactor liquid volume necessary for thermal reactions like resid conversion and thermal-catalytic reactions like Ramscarbon conversion.
Over the years a number of reactors and processes have been suggested for processing oil. Typifying these prior art reactors and processes are those found in U.S. Pat. Nos. 3,124,518, 3,227,528, 3,414,386, 3,677,716, 4,057,397, 4,097,243, 4,221,653, and Re. 25,770. These prior art reactors and processes have met with varying degrees of success.
U.S. Pat. No. 3,414,386, entitled, "Reaction Vessel for Improved Temperature Regulation In Exothermic Reactions" to Mattix, discloses a reactor having a recycle conduit with an enlarged upper end in peripheral contact with the reactor wall to block and prevent fluid flow between the enlarged upper end of the recycle conduit and the reactor wall. The reactor has conduits which extend from a position below the enlarged upper end of the recycle conduit to above the liquid level. Each of the conduits has an overhead deflector plate or cap to deflect the fluids back into the enlarged upper end of the recycle conduit.
U.S. Pat. No. 4,221,653 entitled, "Catalytic Hydrogenation Process and Apparatus With Improved Vapor Liquid Separation" to Chervenak et al., discloses upper portions of downcomers with staggered liquid and gas conduits. Most of the conduits have inlet ends at different levels extending below the upper portions of the downcomer and have outlet ends submerged in the liquid below the liquid level (surface of the liquid).
It is, therefore, desirable to provide an improved hydrotreating process and reactor which overcomes most, if not all, of the above problems.